1. Field of the Invention
2. Description of the Related Art
The present invention relates to a power cable, particularly a power cable for use in a wind mill power plant.
A wind mill comprises a tower and a nacelle on top thereof. The nacelle houses, inter alia, the generator system, the blades and the transformer. The nacelle is suitable for being pivoted (with respect to the tower axis) in order to follow the wind direction changes.
A power cable is positioned to run from the transformer (on the tower top) to the tower base (where the generated electrical power is conveyed to the distribution network or delivered directly to an end user), said cable being vertically positioned along the longitudinal development of the tower, inside thereof.
Typically, the power cable is a tripolar cable and generally comprises three insulated power conductors (each power conductor comprising: conductor+inner semiconductive layer+insulation+outer semiconductive layer) and three earth conductors, each earth conductor being positioned in the interstitial area formed between two adjacent power conductors. The three power conductors and the three earth conductors are helically twisted and the whole assembly is successively coated with a cable outer jacket.
The cable designs known in the art, which are suitable for being used in a wind mill, are typically provided with an outer jacket that penetrates into the interstitial areas present between the earth conductors and the cable power conductors (this is due to the fact that the outer jacket is pressure extruded over the power conductors/earth conductors assembly). Therefore, in the cable designs known in the art, the outer jacket has a thickness which is not constant in the cable cross-section, said thickness being remarkably greater in correspondence of the interstitial areas than at the cable power conductors extrados.
Due to the rotational movement of the nacelle both in the clockwise and in the counterclockwise directions, the power cable is subjected to cycles of alternate torsional stresses. Specifically, the torsional stresses arise in the cable length which is freely positioned within the tower, i.e. the cable length which exits from the transformer and is suspended within the tower before being fixed to the sidewall thereof (said cable length is of about 18-20 m, while the tower height is typically 60-100 m). Generally, a wind mill is operated to make 5 complete turns (360° each turn) in a given direction (e.g. clockwise) and then the rotation is inverted (5 turns in the opposite direction, e.g. counterclockwise). On an average, a wind mill makes one turn/day since the wind direction generally varies not more than 180° in 24 h.
When power cables according to the known art are employed, it may happen that the alternate torsional stresses cause a premature breakage of the earth conductors. Since the breakage of some wires of an earth conductor generally causes a significant variation in the electrical resistance of the earth conductor, the power cable requires to be substituted and the wind mill to be stopped for allowing extraordinary maintenance thereof.